docs:Update 02-context.md (#15700)

* Update 02-context.md

use `const` for key variable and assign it a symbol instead of empty of object.

* Update documentation/docs/06-runtime/02-context.md

---------

Co-authored-by: Rich Harris <hello@rich-harris.dev>

documentation/docs/06-runtime/02-context.md

@@ -94,7 +94,7 @@ interface User {}
 // ---cut---
 import { getContext, setContext } from 'svelte';
 
-let key = {};
+const key = {};
 
 /** @param {User} user */
 export function setUserContext(user) {